Abstract
A long-crack fatigue threshold model which also includes the effect of mean stress or stress ratio R on ΔK 0 in the range −1 to +1 is proposed. The model is based on the hypothesis that near-threshold crack growth can occur by a combination of the micromechanisms of submicroscopic cleavage and reversed shear. The former is controlled by K max and the latter by ΔK, and a transition between the two occurs at a critical stress ratio R crit. The model leads to theoretical ΔK 0 vs. R curves characterizing the mean-stress-influenced fatigue limits of cracked specimens in a manner similar to the well-known Goodman diagrams for uncracked specimens. Excellent agreement of the predicted curves with a large amount of available experimental data for various metallic materials is shown. The model can be used as an alternative procedure for obtaining quick and conservative estimates of ΔK 0 for practical design applications, in preference to crack closure which may be difficult to measure or predict quickly and accurately.
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